The present invention generally relates to wireless communications and, more particularly, to base station antennas for cellular communications systems.
Cellular communications systems are well known in the art. In a cellular communications system, a geographic area is divided into a series of regions that are referred to as “cells,” and each cell is served by a so-called “macrocell” base station. The macrocell base station supports two-way radio frequency (“RF”) communications with mobile subscribers that are geographically positioned within the cell served by the base station. In many cases, each macrocell base station is divided into multiple “sectors,” and different base station antennas, radios and other equipment are used to provide cellular service in each sector. For example, in a common configuration, a base station may be divided into three sectors, and each base station antenna is designed to provide coverage for about 120° in the azimuth plane. The base station antennas may be mounted on a tower or other raised structure, with the radiation beam(s) that are generated by each antenna directed outwardly to serve the respective sector. In some cases, so-called small cell base stations may also be added within a macrocell to provide additional capacity to a small portion of the cell.
Most macrocell base station antennas comprise one or more linear arrays of radiating elements that are mounted on a flat panel reflector assembly. The reflector assembly may serve as a ground plane for the radiating elements, and may also reflect RF energy that is emitted rearwardly by the radiating elements back in the forward direction. FIGS. 1A and 1B are a perspective view and a cross-sectional view, respectively, of a conventional reflector assembly 10. The reflector assembly 10 may be part of a base station antenna. The reflector assembly 10 has a front 12, a back 14 and first and second sides 16. As, can be seen in FIGS. 1A-1B, the conventional reflector assembly 10 may comprise a sheet of metal, such as aluminum, and the front 12 thereof may serve as a main reflective surface 20 that reflects RF energy. Top, bottom and side edges of the sheet metal may each be bent backwardly at an angle, such as a 90° angle. Accordingly, each side 16 of the reflector assembly 10 may have an L-shaped cross-section, as shown best in FIG. 1B. A plurality of openings 22 may be provided in the main reflective surface 20. Various elements of the base station antenna that includes the reflector assembly 10 such as, for example, the radiating elements, decoupling structures, isolation structures and/or structural supports may be mounted in the openings 22. Other of the openings 22 may include attachment structures (e.g., screws, rivets and the like) that may be used to attach various elements/structures to the reflective surface 20.
More recently, base station antennas have been introduced that have reflector assemblies that includes integrated RF chokes. FIGS. 2A and 2B are a perspective view and a cross-sectional view, respectively, of a conventional reflector assembly 30 that includes such integrated RF chokes. The reflector assembly 30 has a front 32, a back 34 and first and second sides 36. The reflector assembly 30 may comprise a sheet of metal, such as aluminum, so that the front 32 of the reflector assembly 30 acts as a main reflective surface 40 that reflects RF energy A plurality of openings 42 may be provided in the main reflective surface 40 that may serve the same functions as the openings 22 discussed above. As shown in FIGS. 2A-2B, the reflector assembly 30 differs from the reflector assembly 10 in that each side 36 of reflector assembly 30 has a U-shaped cross section (see FIG. 2B) as opposed to the L-shaped cross-section of the sides 16 of reflector assembly 10. The U-shaped sides 36 of the reflector assembly 30 form U-shaped channels that run the length of the antenna and act as RF chokes 44. An RF choke is a circuit element that allows some currents to pass, but which is designed to block or “choke” currents in certain frequency bands. The antenna that includes reflector assembly 30 will have one or more linear arrays of radiating elements. Each RF choke 44 (i.e., the U-shaped channels) may have an electrical path length (i.e., the sum of the lengths of each side and the bottom of the U-shape) that corresponds to a 180° phase shift at the center frequency of the frequency band at which one of the linear arrays of radiating elements of the antenna radiates RF energy. Consequently, RF currents that are carried outwardly on the reflective surface 40 may pass down the inner side of the RF choke 44, along the bottom thereof and then back up the outer side of the RF choke 44. As the RF signal at the top of the outer side of the U-shaped channel of the RF choke 44 is about 180° out-of-phase with the RF signal at the top of the inner side of U-shaped channel 44, these signals tend to cancel each other out.